Hypodermic needles are widely used in delivering and withdrawing fluids in medical practice. As originally used, hypodermic needles were used many times, being resterilized between usages. A practitioner would sharpen the needles when they became dull, and then sterilize them prior to the next usage. Since the needles were reused, and often may have needed sharpening, the presence or absence of any lubrication on the outer surface of the needle had little effect on the penetration force or the pain perceived by the patient who was the recipient of the needle. With the development of commercially manufactured disposable needles that always have a fresh well-sharpened point, there was recognition that lubrication of the needle substantially reduced the pain perceived by the patient when a needle was administered to them. A convention is followed in this disclosure wherein the portion of a device toward the practitioner is termed proximal and the portion of the device toward the patient is termed distal.
A tissue penetration by a hypodermic needle involves a sequence of events that collectively are perceived by the patient as whether or not the penetration caused pain. A needle first touches the skin surface, stretches it, the point then cuts into the surface and begins penetration into the tissue. As the shaft of the needle passes through the original cut and into the tissue, there is also sliding friction of the tissue against the needle surface. In the hypodermic needle art when the forces for performing a hypodermic needle penetration are measured, the force measured prior to the needle point cutting the tissue is termed the "peak penetration force" and the force required to continue the penetration into the tissue is called the "drag force." One primary component of the drag force is the sliding friction of the tissue against the surface of the needle shaft. When a subcutaneous or intra-muscular penetration is made with a hypodermic needle, the penetration depth is generally between about 0.5 cm to about 2.5 cm into the patient's tissue. As a result, the practitioner generally does not generally perceive differences in the needle point's penetration of layers. Additionally, most subcutaneous and intra-muscular hypodermic penetrations are made at a relatively high rate (20 cm to 25 cm per second) and utilize the full length of the needle. The rapid penetration rate additionally reduces any perception of layers. The use of lubricant on the surface of hypodermic needles in combination with very well sharpened needles also significantly reduces both the peak penetration force and the drag force. When the reductions by lubrication of the peak penetration force and the drag force of the needle are coupled with the short duration resultant from the high penetration rate, a patient's perception of the painfulness of the penetration is generally significantly reduced. As a result, almost all single-use sterile disposable needles are supplied with a lubricant already applied to substantially the entire needle outside surface. A concomitant effect of the reduction in the peak penetration force and the drag force by lubrication of the needle is a substantial reduction of any ability of a practitioner to discern discrete movement of the needle point through the layers of the tissue.
Hypodermic penetrations are made into the spinal column for the withdrawal of fluid and administration of medicaments, often to induce anesthesia. These spinal penetrations generally utilize needles from about 5 cm to about 9 cm long and the exact penetration depth is critical to the success of the procedure. A commonly practiced technique that requires a hypodermic penetration is delivery of a medicament, generally an anesthetic agent, into the epidural space. When an anesthetic agent is used, this technique results in a regional block anesthesia often referred to as an "epidural." The procedure is recognized by practitioners as being technique sensitive, because the patients are different physical size and a penetration beyond the epidural space and through the dural membrane with or without delivery of the anesthetic agent into the subarachnoid space may result in undesirable consequences. These undesirable consequences can include severe headaches in the postoperative period. Additionally, if the misplacement of the needle is not recognized immediately and a sufficient quantity of anesthetic agent is administered into the subarachnoid space, total spinal block, a potentially life-threatening complication, may result.
Practitioners have developed many ways to determine the placement of the needle in the epidural space. Epidural needles are often supplied with depth markings. Techniques are practiced to identify the epidural space by the loss of resistance to injection of fluid in a syringe coupled to the epidural needle. Another widely practiced technique is the modified drip method, involving an infusion tubing partially filled with normal saline attached to the epidural needle. A technique of hanging a drop of saline on the needle hub is also widely practiced. Any placement of a needle into the spine of a patient is very sensitive to the experience and skill of the practitioner. Epidural needles are generally introduced in the midline at either the lumbar (L) 2-L3 or L3-4 intervertebral space. The needle is slowly and carefully advanced into the skin, encountering subcutaneous tissue, supraspinous ligament and interspinous ligament, then entering the epidural space. The epidural needle is generally used with a stylet occluding the bore of the needle to prevent particles of tissue from being collected in the needle bore and transported into the epidural space with the delivery of the anesthetic agent. The stylet is removed to introduce the anesthetic agent through the needle.
Epidural needles are a notable exception to the general usage of lubricants on commercially available single-use needles, being supplied "dry" or unlubricated. Since the presence of lubricant on the surface of a needle significantly reduces the peak penetration force, if an epidural needle was lubricated at the distal end, the practitioner's ability to perceive the penetrations of the several structures in the tissue, i.e., the skin, subcutaneous tissue, supraspinous ligament and interspinous ligament, is substantially reduced. While the unlubricated needle improves the perception of the needle point penetration through the tissue structures, drag force on the shaft of the needle becomes more significant as the penetration depth increases and, when stick/slip friction is present, the practitioner may not be able to readily determine when the needle point has entered the epidural space.